1. Field of the Invention
The present invention relates to an optical disk and a method for producing the same. In particular, the present invention relates to an optical disk in which a substrate on an incident side of laser light is thinner, and a method for producing the same.
2. Description of the Related Art
In recent years, various studies have been conducted on the recording of optical information in the field of information recording. Recording of optical information can be conducted at higher density, and optical information can be recorded/reproduced in a non-contact manner; therefore, as a method for realizing the recording/reproducing of optical information at a low cost, applications for use in a wide range are being realized. Examples of current optical disks include those having a structure in which an information layer is provided on a transparent resin substrate with a thickness of 1.2 mm and protected by an overcoat or those having a structure in which an information layer is provided on one side or both sides of a transparent resin substrate with a thickness of 0.6 mm, and two substrates are attached to each other.
Recently, in order to increase the recording density of an optical disk, a method for increasing a numerical aperture (NA) of an objective lens, a method for shortening a wavelength of the laser to be used, and the like have been considered. As the thickness of a recording/reproducing side substrate (i.e., substrate on an incident side of laser light) becomes smaller, the influence of aberration on a laser spot can be decreased, and an allowable value of a tilt of a disk can be increased. Because of this, it is proposed that the thickness of a recording/reproducing side substrate, a NA, and a laser wavelength are prescribed to be about 0.1 mm, about 0.85, and about 400 nm, respectively.
In a current DVD (digital versatile disk), mainly, a method is used in which two transparent resin substrates (thickness: 0.6 mm), on which film formation and the like are conducted, are attached, with radiation curable resin. Even when the thickness of a recording/reproducing side substrate becomes about 0.1 mm for the purpose of achieving high density, it is desirable to attach substrates to each other by the same method using the same facility as those currently used.
However, with an optical disk in which two substrates are attached to each other, it is necessary to enhance durability. Furthermore, when the centers of two substrates are shifted from each other, deflections occur when the optical disk is rotated. Therefore, it is required to align the centers of two substrates with each other with high precision. There also is a demand for a method for easily producing such optical disks.